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DONETSK, Ukraine (CNN) — At least 14 people, including a general, were killed Thursday when a Ukrainian military helicopter was shot down by “terrorists” near Slovyansk, acting President Oleksandr Turchynov told parliament.

Turchynov said the chopper, which was carrying soldiers for a troop rotation, was shot down using a Russian rocket-propelled grenade launcher.

Militants in the rebel stronghold claimed responsibility for downing the helicopter, a spokesman for the militants said. The aircraft had flown there from Kramatorsk, he said, where the Ukrainian military has a substantial presence.

The large loss of life will be a major blow to the Ukrainian military, which on Monday toughened its approach to the separatist movement when it launched a fierce assault on militants who’d taken control of part of Donetsk airport.

Also in Slovyansk, the self-declared mayor told CNN that pro-Russia separatist militants are holding four European observers who have been missing since Monday.

But he declined to say where exactly.

“Our militants got them,” Vyacheslav Ponomarev said Thursday. “They were detained because they didn’t respect my request. I asked them not to leave Donetsk (city). They decided they were smarter and could come here.”

Ponomarev added that the monitors from the Organization for Security and Co-operation in Europe were not being held in the town.

They also are not “exchange material,” he said, indicating they were not being held to swap for separatists detained by Kiev authorities.

Ponomarev said he had been in contact with the OSCE, and they were assessing the situation. He said he would likely release the monitors soon, but wouldn’t say when.

On Wednesday, Ukrainian Foreign Ministry spokesman Yevhen Perebynis said a pro-Russia group was holding the OSCE monitors.

“The negotiations for their release are in process,” Perebynis said.

The four team members, who are Swiss, Turkish, Estonian and Danish, were on a routine patrol Monday east of Donetsk city when last heard from, according to the OSCE.

The last time an OSCE team went missing in the Donetsk region, its members also turned up in the hands of the militant separatists in Slovyansk — and were described by Ponomarev as “prisoners of war.” They were freed just over a week later.

There were fears Wednesday that another group of 11 monitors had gone missing after being stopped at a roadblock in Marinka, west of Donetsk, but the group re-established contact with the OSCE after returning to Donetsk, according to an OSCE statement.

Separatists: Bodies to be returned to Russia

Dozens of pro-Russia separatists were killed after Ukrainian security forces launched their assault on Donetsk International Airport on Monday, following the militants’ seizure of a terminal.

The military’s move — only hours after President Petro Poroshenko declared his election win — was interpreted by some as an indication that he will take a tougher stance as he seeks to unify the country.

Now the grim task of returning the dead to their families is under way.

At least 33 Russian citizens are among the militants killed during the violence, a spokesman for the self-declared “Donetsk People’s Republic” told a CNN team at the morgue.

The bodies have been identified and there is an agreement with the local police to escort the remains back to Russia soon, several separatist representatives said.

The bodies will first be taken across the border to the southwestern Russian city of Rostov-on-Don. From there, they will be transported to different cities according to where they are from, reportedly including Moscow in Russia and Grozny in the Russian republic of Chechnya.

The CNN team at the morgue in Donetsk city saw at least 10 coffins.

The separatist spokesman at the morgue said that the men were volunteers from across Russia who had come to help stand with the separatists against the authorities in Kiev.

The spokesman also said that there are still about 15 bodies left at the airport, which the separatists haven’t yet been able to collect or identify. About 20 of those killed are Ukrainian, he said.

The Donetsk mayor’s official website on Tuesday put the airport death toll at 40, including two civilians, but did not specify how many were separatists.

But the separatist spokesman said the death toll could be as high as 70.

If confirmed, this would make Monday the deadliest day in Ukraine since the bloody clashes in Kiev’s Independence Square, or Maidan, which triggered the ouster of pro-Moscow President Viktor Yanukovych and led to the current spiral into violence in the east.

The separatists have until now insisted that their movement is homegrown, so their very public acknowledgment of so many Russian citizens among those killed Monday marks a shift in approach.

It could perhaps be intended to goad Moscow into a response at a time when the separatist movement is under pressure to find a way forward and has shown signs of division within its ranks.

Kiev and the West have accused Russia of coordinating and supplying the separatists, a claim Moscow has denied.

Russia has called for Sunday’s election results to be respected and NATO this week reported signs that some of the tens of thousands of troops Russia has massed by Ukraine’s eastern border may be preparing to pull back.

Clashes in eastern Luhansk

Amid the ongoing tensions, Ukraine’s National Guard base in the eastern Luhansk region was attacked Wednesday by what the country’s Interior Ministry described as “terrorists.”

“There have been losses among military personnel as well as among the attackers,” a statement from the ministry said.

The Luhansk region is, with neighboring Donetsk, at the heart of the separatists’ bid to declare independence from Ukraine.

Eastern Ukraine was a key support base for Yanukovych and many people there oppose the authorities in Kiev, favoring closer ties instead to Russia.

CNN’s Nick Paton Walsh reported from Donetsk and Laura Smith-Spark wrote and reported in London. CNN’s Holly Yan and Andrew Carey contributed to this report.